Fuel cells are comprised of electrochemical devices used for providing an environmentally clean method for generating electricity. What makes fuel cells different from another electrochemical energy converter, such as a battery, is the fact that both fuel and oxidant are continuously supplied to their respective electrodes, and reaction products are continuously removed from the fuel cell. Electric current will continue to flow essentially as long as fuel and oxidant are supplied to the electrodes. Fuel cell systems can be formed by stacking and electrically connecting many electrochemical cells together to provide power generation for residential, commercial and industrial scale power applications. Individual fuel cells in fuel cell systems each include at least two catalytic electrodes in contact with an electrolyte medium comprising an electrode-electrolyte assembly. The individual fuel cells also include devices for managing fuel and oxidant flows thereto and for controlling temperature within operating limits. Use of pure hydrogen gas as a fuel results in higher fuel cell energy density outputs compared to other fuels. However, hydrogen gas has a number of drawbacks including: flammability; storage difficulties; and comparatively high production costs.
Naturally occurring organic fuels as well as synthetic fuels can form hydrogen gas external to the fuel cell system using an endothermic chemical reaction such as steam reforming. Natural gas, propane, and diesel are convenient fuel choices since they are widely available. Processing of natural gas, propane and diesel fuels results in a gaseous mixture of hydrogen and carbon dioxide and reaction intermediates such as carbon monoxide. Steam reforming is limited mostly to steady state fuel cell operations at temperatures much higher than ambient temperature. Various fuel cell designs have utilized steam reformers external to the fuel cell to allow cooling of the fuel prior to use in a fuel cell which operates at near ambient temperatures. Steam reforming outside a fuel cell increases cost and does not provide improved transient load following capability. Hydrogen gas generated by steam reformers external to the fuel cell could be accumulated in a storage facility for subsequent use. However, storage of highly flammable fuels such as hydrogen gas is dangerous. Moreover hydrogen storage facilities generally limit fuel cells to stationary applications.
The performance of fuel cells using catalytic electrodes can degrade due to catalyst deactivation and poisoning by the reaction intermediates such as carbon monoxide (CO), especially near ambient operating temperature. Hydrogen gas produced by steam reforming of hydrocarbons can contain 1.5% by volume or more of CO. For catalytic electrodes comprising platinum, carbon monoxide could be a poisoning intermediate at ambient temperature. Elevation of the operating temperature of the fuel cell to about 200° C. can eliminate such poisoning. While elevating the operating temperature of the fuel cell may be practical in some fuel cell applications operating continuously at or near steady state, it is difficult to implement for applications that use the fuel cell on a transient or as-needed basis and makes the use of polymer electrolyte assemblies impractical. Polymer electrolytes need liquid water in their make-up in order to be electrically conductive and their use in fuel cells is, therefore, limited to temperatures below the boiling point of water. This requires purification of the reformer gas stream to lower its CO content below about ten parts per million (10 ppm). Larger concentrations may negatively impact the fuel cell performance. Typically, purification is accomplished by separate reaction and/or adsorption of the poisoning intermediates. This adds complexity to the system, has a negative impact on system load following capability, adds to the overall cost and reduces system reliability. Modifications of fuel cell electrodes to utilize hydrogen with a higher content of poisoning intermediates include the use of ruthenium in the catalyst on the electrodes, which can lower operating temperature requirements below the boiling point of water. However, fuel cells comprising ruthenium containing catalytic electrodes are more expensive and are typically operated above ambient temperature.
There is a need to provide a fuel cell system including a fuel processing device/system capable of processing fuels with poisoning intermediates, internal to the fuel cell and at or near ambient temperature. Prior art methods and systems for addressing these needs for portable or transient applications were either too expensive, inefficient, or ineffective or a combination of all of these. Based on the foregoing, it is the general object of the present invention to improve upon or overcome the problems and drawbacks of the prior art.